This invention relates to a testing instrument which detects defects in liquid crystal display base plates that are used for liquid crystal display panels, and the like.
When the area and the density of a liquid crystal display panel are to be greatly increased to be suitable for use in a liquid crystal TV, advantage lies in active matrix type liquid crystal display panels. These panels utilize active matrix liquid crystal display base plates further comprising multiple pixel element electrodes installed for each of the pixel elements. The elements are arranged in lines, with gate and source leads installed on each of the pixel element electrodes, and thin film transistors. Commercialization began with relatively small size devices. Typically, liquid crystal display panels of this type are manufactured using an active matrix liquid crystal display base plate, and a transparent base plate, which is placed over the active matrix liquid crystal display base plate and separated by a spacer, then liquid crystal material is sealed in the space between the active matrix liquid crystal display base plate and the transparent base plate.
Many liquid crystal televisions or displays currently in production employ liquid crystal display panels which have 250,000 to 500,000 pixel elements, and some with over 1,000,000 pixel elements.
To build such a large number of pixel elements and the corresponding number of leads on a base plate, various film forming processes are performed inside a clean room where dust is kept at an extremely low level. When the width of the pixel element and the lead is extremely small, presence of trace amounts of micro-dust in the manufacturing environment is directly related to breakage or short-circuit of the pixel element electrodes, gate leads, and source leads. At present, up to 10 of those defects are accepted on an active matrix liquid crystal display base plate, but the product is considered defective if it exceeds this limit.
Present manufacturing technology, however, finds it difficult to reduce these defects within the allowable limit. Consequently, liquid crystal display panels produced with a large number of pixel elements have a high ratio of defective panels, causing high prices of large liquid crystal panels.
Testing methods utilizing mechanical probes have been used to test active matrix liquid crystal display base plates, upon completion. However, because the number of the elements on an active matrix liquid crystal display base plate has become so large that this technique requires a lengthy testing process, and is hence impractical.
No in-line testing has traditionally been conducted on active matrix liquid crystal display base plates until they are completed. A visual check is executed by energizing the finished liquid crystal display panels to verify actual functioning of each pixel element. Defects discovered at this point are difficult and expensive to correct causing the defective products to be disposed of. This is a major cause of the poor yield of active matrix liquid crystal displays.
To solve this problem, a detecting beam is irradiated onto the electro-optical element with an electrical field applied to each pixel element electrode, and the changes in the optical properties of the detecting beam with respect to each pixel element electrode are captured by a camera. The optical changes are converted into a voltage reading for each pixel element, and when the voltage distribution for each pixel element electrode is obtained, defects in the pixel element electrodes, gate leads, source leads, and in other parts on the liquid crystal display base plate which cause display irregularities of the pixel elements, resulting in abnormalities in the voltage distribution, thus detecting the display irregularities of the pixel element electrodes, and therefore, defects are detected.